With the heavy regulation by both federal and state governments concerning in-ground pollution, a need has developed for an economical and effective method of removing pollutants from soil. Gasoline and other petroleum products containing hydrocarbons are one of the largest sources of such soil contamination. Pollution of the soil by hydrocarbons occurs in various ways. For instance, gasoline is generally stored in underground tanks at service stations as well as at refineries and other storage facilities. Before the advent of plastics, underground tanks were made of metals, which over the years were susceptible to corrosion and the inevitable leakage. The Environmental Protection Agency estimates that approximately one-third of the nation's underground storage tanks are leaking. Contamination of the soil also occurs due to spills of oil, gasoline, or other petroleum products or disposal of manufacturing wastes.
Moreover, laws, such as the recently reenacted Superfund, require the disclosure of the presence of soil contaminants to potential buyers of industrial or commercial property. Because no potential buyer would purchase property upon which the soil was contaminated, owners of contaminated sites must rid the soil of contaminants prior to sale of the property.
There are several options in treating soil contaminated with hydrocarbons. The soil can be removed and transported to a landfill for permanent storage. However, many landfill sites will not take soil that is contaminated by hydrocarbons. There are also problems of continuing liability for the tainted soil should the landfill site eventually have to be cleaned up. Moreover, storage at a landfill does not remove contaminants from the soil, nor does it allow the soil to be reused, but merely moves the problem to another site.
Another method of cleaning hydrocarbon contaminated soil is aeration and biodegradation. This process entails placing the contaminated soil on a layer of plastic film and allowing the hydrocarbons to evaporate and biodegrade. This process is very slow, and merely transfers the problem to one of air rather than soil pollution.
A further alternative is incineration of the contaminated soil. While incineration is effective, hazardous waste incinerators are scarce, and incineration is very expensive.
Mobile soil remediation units have also been developed. The mobile unit is attached to a truck and transported from site to site to perform a clean up of the polluted soil. The mobile system comprises a feed hopper which feeds the polluted soil into a dryer having a burner at one end. The tainted soil is heated in the dryer, thus evaporating the hydrocarbons and other combustible materials in the soil. The gases generated by the vaporization of the hydrocarbons are exhausted to a baghouse for removal of particulate matter from the gases. In many instances, a separate afterburner and stack unit is transported along with the unit and attaches to the end of the baghouse. As the gases exit the baghouse, the afterburner heats the gases to destroy remaining hydrocarbons present in the gases. The gases are then exhausted from the stack. The gases emitted from the stack are very hot because of the afterburning. Stationary soil remediation plants embodying the same principles have also been developed.
In the mobile and stationary soil remediation plants having an afterburner, the afterburner accepts exhaust gases after they have exited from the baghouse. This is because in view of current technology, the bag material can only withstand temperatures to a maximum of 350.degree.-400.degree. F. Up to now, if the afterburner were placed before the baghouse, the resulting hot gases would destroy the bags.
However, in some cases, the mobile and stationary systems described above may be too expensive to build, and relatively few are in existence. The apparatus presently in use for mobile soil remediation can also be quite large and cumbersome to transport from site to site depending on the desired volume of soil to be remediated. Furthermore, site permits would be needed, and possibly EPA permits.
Furthermore, if there are vaporized hydrocarbons remaining in the gas stream while in the baghouse, they tend to clog the bags, thereby reducing the efficiency of particulate removal. This also necessitates more frequent changing of bags, and thus greater maintenance costs. Therefore, there is a need for a soil remediation unit which will burn off all of the hydrocarbons in the effluent gases prior to the effluent gases entering the baghouse, but still keeping the exhaust gas temperature sufficiently below the critical temperature 350.degree.-400.degree. F.
A relatively inexpensive and effective alternative method of soil remediation is to utilize an existing hot mix asphalt plant to rid the soil of combustible materials. As opposed to incinerators or landfills, asphalt plants are relatively plentiful as there are approximately 5,000 asphalt plants currently operating in the United States. Processing the contaminated soil using asphalt plants also requires little capital investment as the bulk of the equipment necessary for soil remediation is necessarily already in place. Only certain modifications and additions need to be made to adapt the asphalt plant to process contaminated soil.
A typical hot mix asphalt plant operates as follows. Hot mix asphalt is comprised of aggregate consisting of various sized particulate matter mixed with hot asphaltic oil. Virgin aggregate is loaded into one end of a large drum mixer or dryer drum. It will be understood that while the typical hot mix asphalt plant and the present invention will be described using a drum mixer as an example, the discussion also may relate to hot mix batch plants using a dryer. At one end of the drum mixer is a burner, usually gas or oil-fired. The aggregate is heated in the drum mixer by the burner to evaporate any moisture from the aggregate and to heat it to the mixing temperature of approximately 300.degree. F. Once dry, the aggregate is coated and mixed with heated asphaltic oil either within the drum mixer or, in the case of the dryer, in a separate mixing unit. The inside of the drum mixer has aggregate lifting flights attached. The drum rotates to assure that the aggregate is cascaded through the drying zone, and to ensure total mixing of the asphaltic oil and aggregate. The drum mixer is usually inclined to allow gravity to advance the aggregate and asphalt mix to travel through it. After the hot mix asphalt is mixed, it is then transported by conveyor or elevator to storage silos pending transport by truck to a desired site.
As a result of combustion by the burner, gases are created which entrain particulate matter from the aggregate. These gases are exhausted from the drum mixer. Particulate matter from the gases are removed in a baghouse prior to discharge into the atmosphere through a stack. The operation of a hot mix asphalt plant is well-known to one skilled in the art.
Previous attempts at adapting asphalt plants to perform soil remediation have been made. In early attempts, the contaminated soil was simply added to the aggregate prior to its introduction into the drum mixer. The purpose of this was to use the heat generated by the burner to evaporate the hydrocarbons present in the soil. In many asphalt plants, the aggregate enters the drum mixer at the end opposite the burner. This is known in the trade as a "counterflow" unit. The resulting temperatures at the aggregate entrance were too low to effectively burn off the hydrocarbons, thus causing high levels of hydrocarbons to be emitted to the atmosphere.
Later developments introduced the contaminated soil at the burner end of what is known as a "parallel-flow" drum mixer. See, a speech by Czarnecki, "Processing of Oil Contaminated Soil in a Hot Mix Asphalt Facility." Mr. Czarnecki was employed by Brox Industries, Inc., of Dracut, Mass. The Czarnecki speech discloses a ceramic cylinder added to the burner end of a dryer. The burner is attached to the outside end of the ceramic cylinder, and as a result, the flame from the burner fills the cylinder. Contaminated soil is fed into the ceramic cylinder. The cylinder is equipped with flights and is rotated to allow the soil to cascade through the cylinder while the burner flame heats the soil and burns off the hydrocarbons. The purified soil is then mixed with aggregate and is incorporated into the hot mix asphalt.
This system disclosed in the Czarnecki system is said to result in an efficiency of approximately ninety-five percent hydrocarbon destruction. However, that means that five percent of the hydrocarbons remain uncombusted and are discharged into the air. With this system it is also impossible to adjust for varying types of contaminated soil, or to control other variables. Rather, the parameters for the soil remediation such as burner temperature and residence time in the processing area are set by the needs of the asphalt plant. As a result, the removal of hydrocarbons from the soil cannot be optimized to adjust for variations in the type of soil or hydrocarbon content.